How to position glyphs

[gentlesartori]

Hello,

I am having trouble getting my glyph placement correct. I have tried a few iterations of using x-offset and y-offset, vref, ascent, descent, etc. I guess I don't understand how they all relate, and how they relate to the font positions Baseline, Top, Center and Bottom. I found the code that calculated vref, but using the result has not worked for me. I need to position glyphs correctly in a single line in Baseline and Top modes at a minimum.

As I understand it, the (0,0) position for a glyph defaults to the lower left corner. In Baseline mode, all the glyphs in a string share the same y position (assuming a single line) with a possible downward shift for y-offset. Does Top mode mean that all glyphs in a string have their tops positioned along the same y position? And how do y-offset, ascent and descent factor into these? Finally, how is vref value meant to be used? And what about SetFontRefHeight?

Thanks a lot,

Gentle

[olikraus]

Ascent and Descent are just the standard glyph properties as show here for example: https://stackoverflow.com/questions/5995293/get-single-glyph-metrics-net

Obviously they are in pixel. If the glyph extends below the baseline (like 'g') then the descent is negative. A glyph, which is placed on the base line (like 'a') has a descent value of 0. See also here: https://github.com/olikraus/u8g2/wiki/u8g2reference#getdescent

Nevertheless we need to distinguish between the functions getDescent and getAscent in u8g2.
Let me first discuss the ascent and descent glyph properties:

Ascent and descent are glyph properties. They can't be changed are defined by the font designer.

The x and y offset are part of the compression algorithm. They define how the bitmap is shifted away from the origin.
Usually this should be the last to args of the BBX command in the BDF file: https://en.wikipedia.org/wiki/Glyph_Bitmap_Distribution_Format
Here is a picture from https://github.com/NikolaiPrzewalski (#550):
https://user-images.githubusercontent.com/38105124/38498823-e9eaf866-3c05-11e8-8a8a-690596eb2e56.jpg
So you can say, x/y offset is a vector from the origin to the lower right corner of the glyph bitmap.

The getDescent and getAscent function in u8g2 return something like the overall font values for ascent and descent.
This is useful in order to draw a box around an unknown piece of text.

vref only is used to convert the y position in different modes. vref is 0 for the base line mode (which I suggest should be the default)

In Baseline mode, all the glyphs in a string share the same y position (assuming a single line) with a possible downward shift for y-offset.

Correct. Technically the gylph bitmap starts at the uppler left corner and the y values grow towards the bottom.
To get the bitmap upper left corner we need todo this:

u8g2/csrc/u8g2_font.c

Lines 636 to 637 in 5f96958

	decode->target_x += x;
	decode->target_y -= h+y;

Does Top mode mean that all glyphs in a string have their tops positioned along the same y position?

You can say, that the origin is shifted above the glyph. The shift is the same for all glyph (so it is a font property).
I have a picture for the center mode here: https://github.com/olikraus/u8g2/wiki/u8g2reference#setfontpostop

And how do y-offset, ascent and descent factor into these

y-offset, ascent and descent are gylph properties, the mode (vref) is a font property. It will be like this:

1. x/y provided by user
2. apply vref (same for all glyph s of the font)
3. apply x/y offset (depends on the glyph)

Finally, how is vref value meant to be used?

It is just used by the mode. In baseline mode, vref will be always 0. In top mode, vref is the size of the tallest glyph (well... depends, see next answer)

SetFontRefHeight?

Like noted above, the question is: What is the tallest glphy? The idea of the SetFontRefHeight is to define, the height of the font for the top mode.

So for example with "setFontRefHeightAll" u8g2 will use the height of the tallest glyph found in the font: https://github.com/olikraus/u8g2/wiki/u8g2reference#setfontrefheightall

[gentlesartori]

Good stuff, thanks!

For vref calculation, which values are used to calculate ascent/descent? Is it "ascent (capital A)/descent(lowercase g)"? I think I am confusing myself - I have encountered different labels for values in the font header.

Also, to be sure - for Baseline, vref = 0. Is that correct?

[olikraus]

For vref calculation, which values are used to calculate ascent/descent? Is it "ascent (capital A)/descent(lowercase g)"?

Yes, if setFontRefHeightText is in use. See the description here for more details: https://github.com/olikraus/u8g2/wiki/u8g2reference#setfontrefheightall